1. Field of the Invention
The present invention relates to a device and probe for measuring the variation of distance between the two faces of a layer of material by means of ultrasounds.
The principle of known devices of this type is generally based on the timing of pulses, i.e. the time taken by ultrasonic pulses to travel through said layer of material is measured and the thickness thereof is derived therefrom by multiplying this travel time by the speed of said ultrasonic pulses in the material. These known devices are not very accurate and have poor resolution, because of the uncertainty with which the beginning of each pulse is known.
2. Description of the Prior art
In order to improve such known devices, radar or sonar techniques have been applied thereto by frequency modulating the ultrasonic pulses and filtering them. Then a good signal/noise ratio is obtained and better resolution in time. However, such an improvement means that, in the material measured, the speed of the pulses does not vary with the frequency. In any case, this improvement requires complex processing of the ultrasonic pulses, comprising self correlation and filtering. The result is that, when such processing is digitized, a large number of bits is required processed at a high frequency corresponding to the ultrasonic pulse generation rate. The digital processing device is therefore itself complex. In addition, to obtain ultrasonic pulses, it is necessary to apply high voltages to the electroacoustic transducers used (piezoelectric, ferroelectric, . . . ) which has drawbacks.
To overcome such drawbacks of known pulsed devices, alternating ultrasonic waves have already been used.
For example, the patent FR-A-2 476 831 describes a device comprising:
a generator generating a first alternating electric signal of fixed frequency;
an electroacoustic transmitting transducer able to convert said first alternating electric signal into an ultrasonic wave of the same fixed frequency, whose amplitude varies alternately as a function of time, said ultrasonic wave and said first electric signal being in a fixed phase relation;
first coupling means for applying said ultrasonic wave to said layer of material;
an electroacoustic receiving transducer capable of converting the ultrasonic wave which has passed through said layer of material into a second alternating electric signal in fixed phase relation with said wave;
second coupling means for applying, to said electroacoustic receiving transducer, said ultrasonic wave which has passed through said layer of material;
means for continuously measuring the variation of the phase-shift of said second electric signal with respect to to said first alternating electric signal applied to said measuring means; and
computing means for deriving said variation of distance from said phase-shift variation.
Thus, according to this other known technique, an alternating ultrasonic wave is used continuously, which overcomes the drawback of the known pulsed technique, recalled above, due to the inaccuracy of the knowledge of the beginning of the ultrasonic pulses. In addition, because pulses are not used, the electric voltage applied to the transmitting transducer is not high. Such a technique using an alternating ultrasonic wave gives accurate, reliable measurements which do not require a high number of bits for the computations.
In known devices using this known technique with alternating ultrasonic wave, said layer of material is plunged into a bath of liquid able to transmit said wave and the electroacoustic transmitting and receiving transducers are disposed on each side of said layer of material. Thus, said first and second coupling means are formed by the thicknesses of said liquid between said electroacoustic transducers and the faces of said layer, respectively opposite said transducers.
The result is that such devices are inconvenient in use, since they require said layer of material to be plunged in a liquid bath. In addition, they cannot be used in all cases where it might be desirable to measure the variation of thickness of said layer, particularly when one of the faces of said layer is inaccessible, or when it is subjected to particularly difficult physical and chemical conditions, at the origin of said thickness variation. Moreover, with such a known arrangement, the device is formed with as many discrete devices as it comprises elements.